Nitrogen-based atmospheres have been routinely used by the heat treating industry both in batch and continuous furnaces since the mid seventies. Because of low dew point and virtual absence of carbon dioxide and oxygen, nitrogen-based atmospheres do not exhibit oxidizing and decarburizing properties and are therefore suitable for a variety of heat treating operations. More specifically, a mixture of nitrogen and hydrogen has been extensively used for annealing low to high carbon and alloy steels as well as annealing of non-ferrous metals and alloys such as copper and gold. A mixture of nitrogen and a hydrocarbon such as methane or propane has gained wide acceptance for neutral hardening and decarburization-free annealing of medium to high carbon steels. A mixture of nitrogen and methanol has been developed and used for carburizing of low to medium carbon steels. Finally, a mixture of nitrogen, hydrogen, and moisture has been used for brazing metals, sintering metal and ceramic powders, and sealing glass to metals.
A major portion of nitrogen used by the heat treating industry has been produced by distillation of air in large cryogenic plants. The cryogenically produced nitrogen is generally very pure and expensive. To reduce the cost of nitrogen, several non-cryogenic air separation techniques such as adsorption and permeation have been recently developed and introduced in the market. The non-cryogenically produced nitrogen costs less to produce, however it contains from 0.05 to 5% residual oxygen, making a direct substitution of cryogenically produced nitrogen with non-cryogenically produced nitrogen in continuous annealing and heat treating furnaces very difficult if not impossible for some applications. Several attempts have been made by researchers to substitute cryogenically produced nitrogen directly with that produced non-cryogenically but with limited success even with the use of an excess amount of a reducing gas. The problem has generally been related to severe surface oxidation of the heat treated parts both in the cooling and heating zones of the furnace, resulting in rusting, scaling, and unacceptable metallurgical properties. The use of non-cryogenically produced nitrogen has therefore been limited to applications where surface oxidation, rusting, and scaling can be tolerated. For example, non-cryogenically produced nitrogen has been successfully used in oxide annealing of carbon steel parts which are generally machined after heat treatment. Its use has, however, not been successful for bright annealing of finished carbon steel parts due to the formation of scale and rust.
In the parent patent application referred to above a process for generating low-cost atmospheres inside continuous furnaces suitable for annealing and heat treating ferrous and non-ferrous metals alloys using non-cryogenically produced nitrogen and a reducing gas such as hydrogen, a hydrocarbon, or a mixture thereof was disclosed. The parent application described in detail processes for generating heat treating atmospheres from non-cryogenically produced nitrogen and a reducing gas, particularly hydrogen.